1. Field Of The Invention
This invention relates to analog-to-digital converters, and, more particularly, to spur-free superconductive sigma-delta analog-to-digital converters and multiple flux quanta feedback generators used therein.
2. Description Of Related Art
Sigma-delta converters are a class of analog-to-digital converters. The basic sigma-delta converter utilizes an integrator to which the analog signal to be converted is applied. A single rough comparator operating at high speed converts the output of the integrator to a single digital signal. The comparator is combined with a digital-to-analog converter to form a quantizer. A digital filter converts the high speed single bit output of the quantizer into a multi-bit digital output. The converter also includes a feedback loop including the comparator, the digital-to-analog converter and the integrator. The quantizer samples the integrator output at a rate many times the Nyquist rate. The rate of the multi-bit output signal is a function of the sampling rate and the number of bits in the output signal. The feedback in the sigma-delta converter integrates the error in the least significant bit, thereby shifting quantization noise, which limits the dynamic range of conventional analog-to-digital converters, to frequencies above the frequency of the analog input signal. The chief advantage of sigma-delta converters is that they substitute high speed digital signal processing for the high precision analog circuits required in conventional analog-to-digital converters.
Our commonly owned patent application Ser. No. 07/710,856 filed on Jun. 6, 1991, since issued as U.S. Pat. No. 5,140,324 on Aug. 18, 1992, discloses a sigma-delta converter implemented with superconducting elements and operated with GHz sampling rates to provide high resolution for megahertz signals. This superconducting sigma-delta converter utilizes a superconducting inductor as the integrator, a Josephson junction as the quantizer and Superconducting Quantum Interface Device (SQUID) to generate GHz rate sampling pulses. When the sum of a sampling pulse plus the inductor current generated by the supeconducting inductor exceeds a critical current, the Josephson junction generates a voltage pulse which represents a digital "ONE" output. The voltage pulse also provides feedback to the superconducting inductor. This feedback is very precise and stable as each voltage pulse generated by the Josephson junction is a flux quantum.
Our commonly owned patent application Ser. No. 07/807,040 filed on Dec. 12, 1991, since issued as U.S. Pat. No. 5,198,815 on Mar. 30, 1993, discloses a two loop superconducting sigma-delta analog-to-digital converter which includes a first superconducting inductor to which the analog signal is applied. A resistor converts the current in the first inductor to a voltage which is applied to a second superconducting inductor. The current in the second inductor, which increases quadratically with time, is applied to an overdamped Josephson junction which kicks back a single quantum voltage pulse each time its critical current is exceeded. This voltage pulse reduces the current in the second inductor and serves as a digital "ONE" output. The pulses are also applied to an underdamped Josephson junction in a feedback pulse generator which latches at its gap voltage for the remainder of a half cycle of an AC bias current. This provides a voltage source for the primary of a superconducting transformer having a mutual inductance which provides sufficient flux in the secondary to cause a SQUID to generate in response to each pulse from the quantizer a selected number of feedback pulses which are applied to the first inductor.
Our commonly owned patent application Ser. No. 07/945,803 filed on Sep. 16, 1992 discloses a bandpass sigma-delta modulator for analog-to-digital converters in which an RLC circuit connected to the input analog signal is resonant at an intermediate frequency. A Josephson junction connected to the RLC circuit receives the current flowing through the RLC circuit. The Josephson junction emits a voltage pulse which reduces the RLC circuit current when the current in the Josephson junction exceeds its critical current. Selected multiples of the voltage pulse generated by the Josephson junction are fed back to the RLC circuit. A digital output is generated from the voltage pulses generated by the Josephson junction to complete the analog-to-digital conversion of the input signal.
Although satisfactory for their intended purpose, these modulators have difficulty obtaining 95 dB of Spur-Free Dynamic Range (SFDR) required by certain new applications. The problem with these previous modulators is illustrated in FIG. 1. FIG. 1 shows a prior art modulator 10 having an analog input 12 and a digital output 14. Pulse generator 16 and pulse sharpener 18 create a sampling pulse 20 that is fired at the comparator junction 22 in the quantizer 24. The voltage of the analog input 12 is integrated as current in the sigma inductor 26 in accordance with the following equation: EQU I=.intg.dI=.intg.[V(t')/L]dt'
When the sum of the integrating inductor current and the sampling pulse current exceeds the critical value, comparator junction 22 is forced momentarily into the voltage state. This produces the output data.sub.-- 1 pulse, quenches the current pulse in the sampling input inductor 28, and reduces the sigma inductor 26 current (delta feedback).
When the sum of the currents flowing into quantizer 24 does not exceed the critical value, comparator junction 22 does not pulse. No output pulse appears, data.sub.-- 0. The sigma inductor 26 continues to integrate the input signal. The lack of a kickback pulse against the sampling inductor 28 leaves a slowly decaying current in the loop, which includes the comparator junction 22. Some of this current persists into the next sampling interval.
The persistence of the sampling current into the next sampling interval is shown in FIGS. 2A and 2B. In FIG. 2A, the production of the output data.sub.-- 1 pulse 30 is demonstrated. In this situation, prior art modulator 10 works fine. However, when no output pulse appears as shown in FIG. 2B, the persistent sampling current 32 continues to exist.
The effect of the persistent sampling current is shown in FIG. 3. A JSIM calculation (a SPICE-like Josephson circuit simulator) was used to model a single loop modulator 10 having a dc input voltage which was subjected to 396 sampling pulses. The phase of the sampling junction is plotted, oscilloscope-style, in FIG. 3. Data.sub.-- 1 events caused a 2-pi flip in junction phase. Data.sub.-- 0 events caused momentary changes in junction phase, followed by decay back to the starting point. The traceback from "no flip" goes to a higher phase than the traceback from a "flip", due to the persistent current 32 in the sampling inductor 28.
The direct consequence of the two different starting points is that it is more difficult to get a data.sub.-- 1 following a data.sub.-- 1 than after a data.sub.-- 0, since the traceback from the first data.sub.-- 1 is to the lower starting point. Conversely, this also means that it is easier to get a data.sub.-- 0 following a data.sub.-- 1 that after a data.sub.-- 0. This shift in threshold causes a non linearity in the analog-to-digital conversion process.
This non-linear effect was demonstrated in a longer JSIM calculation. The input was an offset sine wave of 11 microvolts dc plus a 5 microvolt peak-to-peak, 19.5 MHz signal. The FFT of the modulator output data.sub.-- 1s and data.sub.-- 0s is plotted in FIG. 4. The prominent spurs at the n=2, 3, 4, 6, 8, and 9 harmonics were produced by the analog-to-digital converter 10. The ratio of signal power to second harmonic power was only 29 dB. Because of this performance, there is a need for a sigma-delta modulator which reduces this nonlinearity.
Superconductive sigma-delta modulators use single flux quantum (SFQ) pulses for sampling at rates up to 100 GHz and are capable of high-dynamic range analog-to-digital conversion of megahertz signals. Magnetic flux contained in a loop containing a Josephson junction is quantized in units of single flux quantum, .phi..sub.o. This also defines the size of voltage pulses generated by Josephson junctions, the so called SFQ pulses. These pulses are used in digitizing analog electric signals to a very fine scale and in constructing very high speed and ultra low power digital electronics.
Many SFQ circuit elements, operable at above 100 GHz, have been developed in the last few years several circuits have demonstrated operation ar speeds of 100 GHz with a power of tens of nanowatts. Most digital circuits are operated with single pulses and do not require a series of multiple pulses. Devising a circuit which can convert multiple SFQ pulses into an analog signal is useful in constructing many SFQ signal processing circuits, particularly sigma-delta analog-to-digital converters.
To make superconductive sigma-delta modulators useful for practical applications for sampling at rates of up to 100 GHz, two or more feedback loops with multiple pulse feedback are required. The performance of these modulators critically depends on the proper design of the multiple pulse feedback circuit. Previous attempts to simulate proper operation of various multiple pulse feedback circuits have not been successful. Consequently, there is a need for a multiple pulse feedback circuit.